1. Field of the Invention
The present invention relates generally to board testers, and more specifically to an improved tester calibration technique.
2. Related Art
Board testers are widely used in the electronics industry as a tool for board troubleshooting and performance evaluation. These testers are used to perform functions such as integrated manufacturing testing, board diagnostics and troubleshooting, and performance verification.
A goal of tester calibration is making all the resources used for board testing look the same to the tester and to the circuit board under test. More specifically, a goal of tester calibration is to synchronize all signals at the board-to-tester interface with themselves, and optionally, with a reference signal from the device under test.
A specific example is synchronizing test-module-channel timing. Here the goal is adjusting the timing of drivers and receivers of test modules so that signals from all drivers are closely synchronized to one another when they reach the device under test. In other words, a signal generated in the tester should be sent from all drivers at the same time and further, should pass through a test fixture to arrive at the proper pins on the circuit board under test at the same time.
A conventional approach toward synchronizing driver timing used in the industry is to use very high speed reference signals. This approach reduces the skew time between signals but is very costly to implement and the high-speed circuitry (typically ECL) consumes a lot of power.
An additional approach that is commonly used in the IC test industry, but only recently in the board test industry, is a distributed approach. The distributed approach puts an adjustment on each driver itself, thereby allowing each driver to be adjusted individually. Conventional implementation of this approach to date is referred to in the industry as "autocal." Autocal measures each of the digital channels individually via a calibration path within the tester and allows all drivers to be aligned in time with one another.
However, autocal itself is limited. First, each channel is measured via a different path, and as a result, channel-to-channel synchronization accuracy suffers. Second, the channels are only synchronized with one another and cannot be synchronized with a central timing reference of the tester. This leads to a systematic offset with the timing reference and additional systematic errors when multiple timing reference sources are used within the tester. Autocal, therefore, provides a decent initial calibration of the tester but is far from ideal.
Another disadvantage of conventional calibration techniques is that they do not allow calibration of a test fixture. The test fixture is the interface between the tester and the device under test. The test fixture is essentially an adapter, wired to send signals from certain channels of the tester to certain pins of the device under test. Since there are various signal paths within a text fixture, the different propagation delays can cause signals to be skewed at the board interface. For example, multiple bits of an address may arrive at their board destinations at different times due to different path lengths within the fixture.
One conventional approach to minimizing the effects of fixtures on signal synchronization is to design and manufacture the fixtures in such a way that the various signal paths within a fixture are all of the same length. This process is both tedious and costly.